If water reaches his throat because of rinsing his nose when fasting

Firstly: 

The fasting person is
forbidden to go to extremes in rinsing his nose, because the Prophet
(peace and blessings of Allaah be upon him) said to Laqeet ibn Sabirah (may
Allaah be pleased with him): “Do wudoo’ well, make the water run in between
your fingers and toes, and go to extremes in rinsing the nose, unless you
are fasting.” Narrated by Abu Dawood (142) and al-Tirmidhi (788); classed as
saheeh by al-Albaani in Saheeh al-Tirmidhi. 

This hadeeth indicates that
one should avoid going to extremes in rinsing the nose when fasting, so that
the water will not inadvertently enter the fasting person’s body. 

Secondly: 

If the fasting person
rinses his mouth or nose and some of the water goes down into his throat
without him intending it to, that does not break the fast, because Allaah
says (interpretation of the meaning): 

“And there is no sin on
you concerning that in which you made a mistake, except in regard to what
your hearts deliberately intend”

[al-Ahzaab 33:5] 

This person did not
deliberately intend to do that which invalidates the fast, so his fast is
still valid. 

See al-Sharh al-Mumti’
(6/240, 246). 

See also question no.
40698. 

And Allaah knows best.